DLL reference disappears (and says "Type Not Defined") from my project Object Browser when I compile?

I wrote a VS2010 Class Library to print Crystal Reports. I want to call that dll from a VS2010 windows project. I include the dll as a reference and can see it in the Object Browser (with all public properties available)

BUT.... when I try to compile I get an error saying "Type Not Defined", and it disappears from the Object Browser (still there as a reference)

Anyone know why this would happen? And what I can do to fix it?

thanks

Ray